Method of making a coupling for rigid pressure pipe

ABSTRACT

A coupling comprises a body of fiber glass reinforced plastic which has a rubber sleeve embedded therein. The rubber sleeve is a one piece unit which includes specially shaped sealing gaskets on either end and has a rubber liner spanning the area between these sealing gaskets. This rubber sleeve is stretched onto a steel and plastic molding mandrel. This stretching places the rubber liner under tension which reduces the cross-sectional area of the liner and compensates for thermal expansion of the liner during curing. The mandrel/sleeve combination is rotated in order to wind a thermosetting resin impregnated fiber glass roving around the outer circumference of the mandrel/sleeve combination, thus embedding the sleeve in the fiber glass reinforced resin matrix. The thermosetting resin is cured at elevated temperatures, hardening into a rigid coupling with the embedded rubber sleeve integral therewith. Thereafter the hardened coupling is stripped from the steel and plastic mandrel and subjected to a further curing and finishing operation.

This is a division of application Ser. No. 918,226, filed June 23, 1978.

BACKGROUND OF THE INVENTION

This invention relates to a novel coupling made of a composite of fiberglass reinforced plastic and an integral rubber sleeve for use ininterconnecting sections of essentially rigid asbestos-cement pipe forforming a conduit system for carrying fluids under pressure. In the pastsuch systems have in general included lengths of asbestos-cement pipewith precisely turned or machined ends. These ends were interconnectedby a coupling made from a relatively large thick walled section ofasbestos-cement pipe. This coupling was made by cutting a length of pipeof proper dimensions into a series of relatively short cylinders, andsubsequently machining a sealing ring groove into both ends of theinterior diameter of each short cylinder of asbestos-cement pipe. Whilesuch a coupling system incorporated all the structural, chemical andavailability advantages inherent in the use of the asbestos-cementmaterial, forming, in general, an entirely acceptable pressure conduitsystem, the great number of machining steps needed to form the couplingitself from an asbestos-cement pipe resulted in many drawbacks. Effortsto reduce or eliminate airborne particles of asbestos which result fromthe various cutting and machining operations necessary to form thecoupling have led to concomitant increase in the cost and complexity ofthese operations. These various machining operations are accomplishedunder a flood or spray of water to hold the minute particles ofasbestos-cement and prevent their loss to the atmosphere. However,increased restrictions on the discharge of this asbestos bearing watereffluent makes this solution less economically viable.

The coupling itself tended to be large and massive. Manipulating thismassive coupling in the field often resulted in improperly connectedpipe Also, the coupling required that the trench be enlarged in the areaof the coupling in order to accommodate its greater diameter. Also, thesealing rings, which were positioned in the machined grooves at eitherend of the coupling, were in constant danger of being displaced duringthe connection of the spigot ends of the pipes into the coupling.

Attempts in the past to use so-called advanced materials as a substitutefor this asbestos-cement coupling have largely met with varying degreesof failure. Each such attempt has incorporated one or a number of theabove enumerated disadvantages inherent in the asbestos-cement coupling,or have incorporated into the system certain disadvantages inherent withthe substitute material itself. One such inherent disadvantage has beenthe result of the wide discrepancy between the modulus of elasticity ofthe asbestos-cement pipe lengths intended to be sealingly connected bythe coupling and the lower modulus of elasticity of the material used inthe coupling itself. This differential in rigidity between the pipe andthe coupling can cause the overall system to fail and leak, especiallywhen there is less than optimum placement of the pipe in the trench. Forexample, two pipe lengths which are intended to be connected by asomewhat less rigid ring may shift relative to one another when the fillunder one such length of pipe is less compacted than the fill under theother length of pipe. Under this condition the weight of one length ofpipe is borne by the relatively elastic coupling. The coupling reacts tothis stress by deflecting and resulting in a leak between the sealinggasket and one or the other of the pipe lengths. While such deflectionand concomitant leaking is less critical and indeed less likely tohappen in a nonpressurized conduit system, this deflection andconcomitant leaking can have disasterous results in a pressurizedsystem.

Another known coupling was constructed of a fiber glass reinforced epoxybody having a pair of separate gasket seals positioned at each end ofthe inner diameter. These gaskets were embedded in the fiber glass/epoxymatrix, but required a separate layer of urethane rubber which wassprayed onto the mold mandrel and the individual gaskets to form a linerbetween the gaskets and to form the bond between the gaskets and thematrix. Also, the cost of epoxy resin and the curable urethane rubbermade the coupling too expensive.

U.S. Pat. No. 3,462,175 shows a filament wound coupling which is formedon a cylindrical mandrel with removable rubber inserts which formtapering spiral contours on the inner diameter of the fiber glass/matrixbody. But this system requires assembly in the field and the rotaryengagement with the pipe ends would be impractical for large diameterpressure pipe.

Another publication of interest is Japanese Pat. No. 61,683 patentedNov. 10, 1924, which shows a connecting coupling having sealing gasketsand an integral connecting liner held in sealing engagement to the endsof pipe sections by steel rings.

Hence, it would be highly advantageous to construct a coupling whichwould utilize the benefits of the so-called advanced compositematerials, such as fiber glass reinforced plastic, permitting theconstruction of a relatively light, strong coupling, and yet wouldresult in a pressurized conduit system which would tolerate the stressesnormal to subterranean installations. Such a system would be desirably aone-piece system to permit easy installation of the couplingarrangement, be inexpensive to manufacture, withstand the rigors ofcarrying fluid under elevated pressure and be relatively cheap tomanufacture.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a coupling forinterengaging lengths of rigid pipe to form a pressurized fluid conduitwherein the coupling ring is composed of an integral construction offiber glass reinforced plastic and a fluid tight rubber sleeve havingintegral therewith sealing gaskets firmly embedded in said fiber glassreinforced plastic for engagement with said rigid pipe.

It is another object of the invention to provide a system formanufacturing such a coupling in an economic and dependable manner.

It is another object of the present invention to provide a mold mandrelwhich is easily and inexpensively constructed, and which permits precisecontrol of the dimensions of the coupling constructed thereon,especially those dimensions critical to the sealing function of saidcoupling.

The instant invention provides such a coupling by forming a rubbersleeve from a continuous length of polyester resin compatible rubbersuch as EPDM (peroxide cured ethylenepropylene-diene terpolymers)rubber. Critical dimensions of the pipe engaging sealing portions andfiber glass reinforced body engaging surfaces of the rubber sleeve arecontrolled by placing the sleeve under a predetermined tensile stressduring the formation of the fiber glass reinforced polyester bodyportion of the coupling. Such a pre-stress condition is obtained byforming the liner in a predetermined circumference such that the sleeveis stretched around a generally cylindrical mandrel prior to theformation of a plurality of layers of thermoplastic impregnated fiberglass roving about its outer circumference. Preferably the centralportion of the rubber sleeve which interconnects the sealing gasketportions along each lateral edge is supported during the fiber glasslay-up and curing operation by a removable generally cylindricalpolymeric support, such as polyethylene, in order to stretch thisportion of the sleeve the required amount and to hold this portion ofthe sleeve in approximately its final configuration. The rubber sleeve,held in this pre-stressed condition can be safely subjected to therelatively elevated temperatures of the curing cycle inherent in theexothermic reaction characteristic of the thermosetting plastic curingoperation as well as the oven treating operation necessary to obtain auniformly cured condition. Upon cooling the sleeve has relaxed due tocreep at the curing temperature, thus relieving the longitudinal stressprovided by the mandrel and central polyethylene support. The rubbersleeve assumes approximately the proper undeflected state relative tothe now rigid fiber glass reinforced plastic matrix. In such a state themechanical and/or chemical bond between the rubber liner and the fiberglass reinforced plastic matrix remains in an optimum condition assuringthe maintenance of proper dimensional tolerances during installation andservice life of the coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of the completed coupling according tothe instant invention.

FIG. 2 is a sectional view of the coupling along line 2--2 of FIG. 1 inoperative engagement with a pipe section.

FIG. 3 illustrates the overall process for forming the coupling ring.

FIG. 4 is a partial sectional view of the completed coupling on aforming mandrel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As can be seen from FIG. 1 the coupling of the instant invention in theform of a generally annular body which consists essentially of two mainparts. An outer fiber glass reinforced plastic body portion 10 and arubber sleeve 12 which constitutes a major portion of the interiorsurface of the coupling 1.

FIG. 2 shows a cross-sectional view of a coupling with an end 4 of pipesection 2 thrust into one end of the coupling to show theinterengagement of the coupling with the pipe section. The rubber sleeve12 consists of three main sections, gasket portions 30 and 30' at eitherperipheral end and a liner portion 36 spanning the annular area betweengasket portions 30 and 30'. Each gasket portion 30, 30' includes a lipportion 32 or 32' respectively which bears against corresponding annularportion 8 of pipe section 2 and which is adapted to be forced inwardlyby pressure within the coupling into firm engagement with annularportion 8. Between finned portions 32 or 32' and liner 36 is an annulargroove 34 or 34'. As can be seen in the right hand portion of FIG. 2 thefunction of 34 and 34' becomes most apparent. When end portion 4 of pipesection 2 is thrust into engagement with the coupling the relativediameter of annular portion 8 is such that gasket 30 and finned portion32' are forced radially outward and are deformed between thecorresponding portion of plastic body 10 and annular portion 8. Thedisplacement of the elastomeric material making up gasket portion 30' isin an axial direction since radial expansion is prevented by body 10.Annular groove 34' provides space for this axial flow. Without such aspace the amount of force required to install end portion 4 into thecoupling would be considerable, resulting in unacceptable radial stresson the coupling 1 and making the insertion of end 4 into the couplingvirtually impossible.

Axially inward from gasket portions 30 and 30' is rubber liner portion36. As illustrated in FIG. 2, this portion includes annularcircumferentially extending interlocking ribs 28. These ribs are of aknown type and are used to mechanically interlock liner portion 36 withthe central portion 15 of fiber glass reinforced body 10. Ribs 28, whilenot necessary to the basic functioning of the connecting ring,supplement any adhesive bonding of the rubber liner 36 to thethermosetting plastic constituent of body 10.

Gasket portions 30 and 30' include axially extending circumferentialprotrusions or ribs 24, 24' and 26 and 26'. These ribs function in asimilar manner to interlocking ribs 28 of liner portion 36 but are morevital to the proper operation of the instant invention. Ribs 24, 26,etc., in conjunction with the radially outwardly facing portion ofgasket 30 define an enlarged ring of elastomeric material which becomesembedded in the thermosetting plastic matrix of body 10. This embeddingaction prevents displacement of ring 30 which might otherwise resultfrom the axial thrust on radially inward portion of 30 and lip 32resulting from the installation of the end portion 4 of pipe section 2as well as the axial thrust resulting from shoulder portion 6 of pipesection 2 bearing against gasket portion 30.

This embedding action can be seen when one considers for example theoperation of shoulder portion 6 as shown on the right hand portion ofFIG. 2. An axial thrust to the left of FIG. 2 would bring shoulderportion 6 firmly against gasket portion 30'. While gasket portion 30'remains under radial outward compressive stress provided by the fit ofannular portion 8 on pipe 2, this axial thrust by shoulder portion 6would begin to drag the contacting edge of gasket portion 30 in theaxial direction which would stretch the bulk of the rubber material,relieving this outward compressive stress and in fact placing that partof 30 near 24' in tension. This tensile force, if unopposed, wouldunseat 30 from its embedded condition in body 10. Such unseating wouldbe undesirable in that it would permit end portion 4 to move intocontact with the corresponding end portion of an adjoining pipe section.

But, more importantly, such unseating may occur by inserting end 4 intothe coupling at an angle such that the leading annular edge of 4 impactson the shoulder engaging edge of gasket portion 30'. If such unseatingtook place in the field, resulting in a "fish mouthed" gasket, thecoupling would be damaged and not seal correctly.

However, both of these modes of failure are substantially eliminated byincorporating annular protrusions 24 and 24' since they, in combinationwith narrow portions 35 and 35', the adjacent part of gasket portion 30and 31 as well as opposite annular protrusions 26 and 26', provide amass of rubber which is substantially surrounded by fiber reinforcedplastic body 10.

A similar case can be made for annular enlargements 26 and 26' exceptthat, in addition to cooperating with 24 and 24' as outlined above, theyoperate in a similar manner to resist unseating forces which affectgasket portions 30 and 30' from the opposite axial direction (i.e. tothe right in FIG. 2 for gasket portion 30'). These unseating forces areunlikely to occur during the coupling operation as outlined above, butare more likely to occur during the service life of the pipe/couplingsystem when for example adjacent pipe lengths shift relative to oneanother.

Surrounding the elastomeric sleeve 12 is fiber glass reinforced plasticbody 10. Extending axially on either end of 12 are portions 11 and 11'of body 10. These portions define frusto-conical guiding surfaces 13 and13' which operate to guide end portion 4 of pipe section 2 into properengagement with gasket portions 30 and 30' as well as protect the gasketportions 30 and 30' from mechanical displacement and abrasion duringshipping, storage and installation. Central portion 15 of body 10supports sleeve portion 36 and has sufficient mechanical strength towithstand the mechanical and hydrostatic loads associated with thefunctioning of the coupling 1. In general, the overall structure of body10 consists of circumferential and helical wraps of fiber glass rovingimpregnated with a thermosetting plastic, preferably a knownpolyester/hardener system. In addition to these circumferential andhelical fiber glass reinforcing layers, body 10 includes axial fiberglass reinforcing 17 which is embedded substantially in the middle ofthe volume of reinforced plastic making up body 10. In the preferredembodiment, this axial reinforcing consists of about two layers of fiberglass "scrim"; a fabric made of discrete bundles of fiber glassfilaments oriented parallel to the axis of the coupling, these bundlesbeing held parallel with one another but spaced a uniform distance byhighly spaced strands of polyester thread. This product is made forcommercial sale by the Erskin-Johns Corporation.

The method of making the coupling illustrated in FIGS. 1 and 2 can bestbe seen with reference to the schematic diagram of FIG. 3. In thispreferred method the elastomeric sleeve 12 is cut from a continuousextrusion of peroxide cured EPDM rubber. The choice of this particularcompound will be more fully explained later. The cut section of theextrusion is spliced to form the desired annular configuration. It isnecessary that the completed annular sleeve 12 have sufficientmechanical integrity at the splice so as to not break or split undercoupling/pipe assembly and operational stresses. It has been found thatthis particular rubber extrusion can be spliced by coating the ends witha rubber cement supplied by the manufacturer and vulcanizing this jointin a heated vulcanizing mold which conforms substantially to the contourof the sleeve 12. Such vulcanizing mold must be able to maintain thesplice under pressure so as to hold the cemented abutting ends inintimate contact during heating. Also, the mold should impart a smoothsurface to the sleeve at the gasket portions to prevent a possiblesource of leakage when engaged with a pipe end. The particular finalsize of the sleeve 12, especially its circumferential dimension, isquite critical to the correct operation of the overall coupling.Obviously this circumferential dimension is determined at the time ofcutting the extrusion and splicing. It has been found that during curingof the polyester body after it has been formed onto the rubber liner,the exothermic reaction generates temperatures as high as 340° F. to360° F. at the interface 14 between the liner 12 and body 10. Thiselevated temperature causes the sleeve 12 to expand volumetrically,causing a lengthening along its circumferential dimension as well as anincrease in its cross-sectional area. This expansion in cross-sectionalarea is most critical since such expansion would cause an increase inthe effective diameter of the fiber glass/polyester body in the areasupporting sealing gaskets 30 and 30'. It must be remembered that due tothe relatively flexible nature of the coupling during its operation insealing to a pair of rigid pipe sections, any deviation from the optimumdiameter in this area can cause leaks between these gaskets and thecorresponding sealing surfaces of the pipe sections.

Hence, in an effort to correct for this inherent volumetric expansion ofsleeve 12 the splice as outlined above is made so as to define a sleevehaving a circumferential dimension which is approximately 2% to 3% lessthan the circumferential dimension of the sleeve in its final embeddedcondition in the finished coupling 1. The spliced liner is theninstalled on a rotatable mold mandrel in such a manner so as to bestretched to its actual final circumference. This stretching operationcauses the cross-sectional area of the sleeve 12 to be reduced to suchan extent that the thermal expansion during the exothermic curingoperation does not cause the cross-sectional area at the resultingelevated temperature to displace the curing resin body and causedimensional discrepencies in the completed coupling. Put another way,presuming that the initial circumferential dimension of the sleeve 12 is2% less than its final configuration, when placed on the properly sizedrotatable mold mandrel it will be stretched along its circumferentialdimension by approximately that 2%. The volume of the mass of rubbermaking up sleeve 12 remains essentially constant since the volumecompressability of rubber is almost zero. Accordingly, itscross-sectional area must be reduced by an amount determined by thePoisson's ratio of rubber, which, for extensions of this magnitude,equals about 0.5. This results in an average reduction of thecross-sectional area of about 1% from the unstretched condition. Thecoefficient of thermal expansion for rubber is between 8×10⁻⁵ /°F. and12.5×10⁻⁵ /°F. While the exact mechanism is not fully known, it islikely that the linear expansion of between 2% and 3.5% (based on theabove coefficient range) experienced by the sleeve 12 during theexothermic heating is substantially compensated for by this 1% initialreduction in cross-sectional area resulting from the initial stretchingstep. In any event the dimensional aspects vital to the sealingoperation are properly controlled.

The exact degree of circumferential stretching for all couplingdiameters is not necessarily 2%. This figure seems to at least apply tocouplings used for 24 inch nominal diameter pressure pipe. The followingcriterion however would hold for virtually any size coupling madeaccording to the disclosed method.

The upper limit on the amount of circumferential stretch is definedprimarily by the difficulty in mounting the sleeve, together with thesupport 38, onto the mandrel 50. For example, splicing the sleeve inorder to require stretching the sleeve by 10% would make the manualmounting of the sleeve on the mandrel virtually impossible whilebenefiting the dimensional control aspects little, if at all, over anominal 2% stretch. Conversely, a 1% stretch may make the mountingoperation quite easy, but would be inadequate to control the finalgasket dimensions.

It could be argued that this pre-stretching could be eliminated bymerely allowing for this thermal expansion and subsequent distortion ofbody 10 when constructing the mandrel 50 that is, if it were found thatthe thermal expansion of an unstretched sleeve results in an increaseddiameter at gasket portion 30 of 0.050" for example, one need only makethe mandrel 50 0.050" smaller in diameter to compensate. However, thismethod of correction does not give the reproducible results required.Unlike the pre-stretch method, this method does not dependably andfirmly seat the sleeve on the mandrel, prevent shifting of the sleeveduring the fiber glass filament winding operation, nor does it takeadvantage of the effects of thermal creep as the pre-stretch method doesas outlined above.

With the elastomeric liner 12 properly installed on the cylindrical moldmandrel the process of building up the polyester body 10 is begun. Themandrel with liner 12 is rotated while a length of fiber glass rovingimpregnated with a polyester-hardener system is caused to wind onto theouter circumference of the mandrel/liner arrangement. The preferredsystem for accomplishing this impregnation operation is disclosed inU.S. Pat. No. 4,068,619 issued to Robert Lee Batts on Jan. 17, 1978 andassigned to the Assignee of the present application. The particularpolyester material is of a known type and obtainable from FreemanChemical Company. This material is combined with approximately 2% ofcumene hydro peroxide (C.H.P.) hardening agent as it is added to thereservoir in the glass fiber impregnating apparatus disclosed in theabove-referenced patent.

The particular fiber glass reinforcing material is subject only togeneral considerations of supply, ultimate structural strength of thebody 10 and compatibility with the polyester matrix material. In thepreferred method, 23 to 27 strands of fiber glass roving material aredispensed from a creel arrangement of known design and passed throughthe impregnating apparatus. The fiber glass material which has beenfound to be acceptable is an item of commerce obtainable fromOwens-Corning Corporation, and is known as Type 30 and is preferablydispensed from a coreless package of continuous roving designated as"TRANSPACK". The fiber glass roving strands are treated with a silicatetype sizing which is compatible with the polyester matrix material andpermits quick and thorough saturation of the filaments making up theroving.

The particular sequence involved in applying the fiber glassreinforcement material is as follows. Generally the first layer of fiberglass roving is comprised of a series of circumferential windingsspanning the entire axial length of body 10. It is preferred that aplurality of circumferential turns be applied radially inward from andin close abutting relation to each annular projection 24, 26, 24' and26'. At least one circumferential turn of roving should be appliedbetween each circumferentially extending interlocking flange 28. Thiscircumferential reinforcement places the fiber glass in close proximityto those portions of rubber liner 12 which require mechanical strengthin order to prevent the liner 12 from being displaced from its embeddedrelation with body 10.

Upon completion of the circumferential wraps as outlined above, a seriesof helical wraps are applied. These helical wraps are continued untilthe entire outer surface of the rotating mandrel is covered. At thispoint the axial glass reinforcement 17 is applied. This reinforcementconsists of two circumferential wraps of the above designated type ofscrim. This scrim need not be preimpregnated with the polyester/hardenermaterial since an adequate amount of this matrix material is carried tothe rotating mandrel on the glass roving. It has been found necessaryonly to hold down the scrim 17 with a circumferential wrap of theimpregnated roving in the middle and at both axial ends of the coupling.The position of the axial ends of scrim 17 should be such that the areaproximate to and axially out from protrusions 24 and 24' (shown in FIG.2 as shear plane S) is avoided. It has been found that placement nearshear plane S may cause a delamination in this area when thecoupling/pipe system is subjected to elevated hydrostatic pressures.

Finally, a second series of helical wraps are applied over the axialreinforcement 17 and completely cover the outer circumference of therotating mandrel.

At this point the mandrel is removed from the winding apparatus andtransferred to an infrared curing oven. The mandrel continues to rotateduring this initial infrared curing since the polyester matrix is quitefluid and would tend to drip or form some asymmetrical shape if themandrel were allowed to remain static in either the vertical orhorizontal position. This infrared cure step progresses forapproximately 20 minutes, or until the polyester matrix has adequatephysical integrity to permit the removal of the mandrel. It is alsoduring this time that the sleeve 12 experiences most of the thermalexpansion and relaxation or thermal creep. Hence, the criticaldimensional aspects are substantially stabalized before the mandrel isremoved, and remains relatively unaffected by subsequent processingsteps. It should be remembered that, despite the circumferentialstretching during mounting on the mandrel, the sleeve has little if anytendency to return to its original size, but due to the acceleratedcreep during the exothermal reaction, has rather assumed the desiredfinal diametrical size.

The mandrel is now removed and prepared for the next winding operationwhile the completed and partially cured coupling is transferred to a hotair oven for its final curing step. This final curing operation lastsabout one hour and in the preferred embodiment the temperature in theoven is at about 175° F. The coupling at this point is substantiallycompleted, requiring only that the sharp edges or flashing which mayexist on the extreme axial ends of the coupling ring be removed bysanding or otherwise.

FIG. 4 shows the detail of the rotatable mold mandrel used in formingthe coupling. The mandrel 50 has a main body portion 52 which isessentially a smooth cylindrical steel body which includes end portions54 extending from the cylindrical portion 52 to a hub (not shown) forconnection to the various apparatus for rotating the mandrel duringwrapping, curing, etc. Cylindrical portion 52 preferably has a veryslight taper (perhaps 0.005 inch difference from one end to the otherfor a nominal 24 inch diameter coupling) in order to permit easy removalof the finished coupling. On either end of cylindrical portion 52 areend rings 56 and 56' which are held in position by rotatable bars 57 and57'. End rings 56 and 56' define the extreme axial ends of body 10 andform the molding surfaces for those surfaces as well as thefrusto-conical guide portion 13 as outlined supra. Also, the extremeinward portion of these end rings support and stretch the gasketportions 30 of elastomeric liner 12. Positioned between cylindricalportion 52 and sleeve portion 14 of sleeve 12 is mold insert 58 whichsupports liner portion 36 in its circumferentially stretched conditionas outlined supra. Insert 58 is preferably a generally rectangular pieceof relatively stiff, dense polymeric material, such as linearpolyethylene, having a length substantially equal to the stretchedcircumferential length of the sleeve 12. Along each circumferential edgeof insert 58 are gasket abutting edges 59, and undercut edges 60. Theinsert 58 is preferably positioned in the spliced sleeve 12 and then theinsert/sleeve combination is forced onto the cylindrical portion 52 ofthe mandrel by hand. Subsequently one or the other of the end rings 56or 56' is positioned over its corresponding end of cylindrical portion52 and fastened with toggles 57 or 57'.

The preferred construction of the mandrel 50 has many benefits inherenttherein. First, the overall construction is merely one of a rigid singlepiece cylinder not requiring complex interconnecting mold halves,precisely turned surfaces of complex nature, etc., which are normallynecessary in order to permit releasing of the hardened polyester body 10therefrom. Also, since most of the internal surface of the coupling ringis defined by the elastomeric lining 12, only end rings 56 and 56'require any precisely contoured surfaces. The use of the polyesterinsert 58 eliminates the need of constructing cylindrical surface 52with any raised portion to support the liner portion 36 of the sleeve12. This permits insert 58 to slide axially with the completed and curedcoupling 1 when it is removed after end ring 56 has been removed. Insert58 can of course be peeled from the inside of the cured coupling ringfor subsequent insertion in the next sleeve 12. During the mounting ofthe sleeve 12/insert 58 combination onto mandrel 50, abutting edges 59assure that the gasket portions 30, 30' and lip portions 32 and 32' arecorrectly positioned on the corresponding part of end rings 56 and 56'.However perhaps more importantly, edges 59 in conjunction with undercutportions 60 further aid in preventing gasket portions 30 and 30' fromthermally expanding in a manner so as to distort the body 10 duringcuring. During this stage of the manufacturing operation as statedsupra, sleeve 12, and more specifically gasket portions 30 and 30', havea tendency to not only expand against the curing resin matrix radiallyoutward therefrom, but also to expand in the axial direction of mandrel50. Annular cavity 34 provides relief for some of this axial expansionby providing an empty space into which parts of the thermally expandingrubber can move. While this relief provided by cavity 34, in combinationwith the circumferential pre-stressing set forth supra, corrects formost of the deleterious effects of this thermal expansion, it was foundto generate a distortion in the final position of the gasket portion 30.This distortion seemed to be the result of lip 32 being held immovablein the slot-like juncture between the immediately adjacent supportingportion of end ring 56 and the abutting edge of support 58. Anyexpansion along a line between projection 24 and lip 32 translated thebulk of gasket portion 30 in the direction of 24, destroying the precisepositioning needed to achieve the sealing function in service.

However, the provision of undercut 60 permits lip 32 to expand axiallyrather than to thrust gasket 30 in the direction of projection 24,acting as a safety valve for this thermal expansion. Thus, abuttingedges 59 contrive to initially position and to hold the bulk of gasketportions 30 and 30' on end rings 56 and 56'. Undercut portions 60 aresuch that they do not detract from this vital operation of support 58,but are remarkably effective in balancing the thermal distortion asoutlined supra. These undercut portions are shown slightly exaggeratedin the drawings. Preferably, however, they form an angle of about 20° toabutting edges 59. It has been found that without this edge feature oninsert 58, the distortion of critica 1 diameter at gasket portions 30and 30' resulted in a 60% dimensional rejection rate. However, uponapplication of inserts 58 having edge features 59 and 60, the rest ofthat production run was within tolerances.

As stated previously, sleeve 12 is preferably formed from a peroxidecured ethylene-propylene-diene terpolymer type rubber which isfrequently referred to by the ASTM designation "EPDM". It has been foundin the past that under certain conditions, for example when a polyesterresin which uses a peroxide curing agent is cured in contact with arubber compound having a highly reactive surface, for example a sulfurcured rubber compound, that the polyester resin will not fully cure inthe presence of such rubber and the resin remains tacky in the area ofthe rubber/resin interface. The insufficient curing thus leaves theresin article badly weakened in the area of the interface and alsoprevents the rubber body from being properly retained and/or adhered tothe resin body. In a pipe coupling such as disclosed in the instantapplication, such insufficient curing of the resin creates a weakenedarea in the pipe coupling wall and also prevents the rubber gasket frombeing satisfactorily retained within the structure, particularly whenforces are exerted against the rubber gasket during joining of adjacentlengths of pipe in the field using such couplings. In addition, theuncured resin itself acts as a lubricant and allows the rubber gasket tobe pulled easily out of the coupling.

The polyester resin of the matrix portion of body 10 may be anypolyester resin in which curing of the resin is initiated by a peroxide.Descriptions of typical polyester resins, the curing reactions, and theuse of peroxide curing agents ("initiators") are widely found in theliterature; a typical description is found in Noller, Chemistry ofOrganic Compounds (3d edn., 1965) at page 885.

The surface affinity of the peroxide cured EPDM rubbers for the peroxidecuring agents of the polyester resin is inherently sufficiently low thatwith conventional amounts of curing agent in the uncured resin acomplete cure can be accomplished using conventional reactionparameters. It should be noted, however, that the sleeve 12 is formedfrom a commercial peroxide cured EPDM rubber body made by extrusion andthat during the extrusion process it will be coated with extrusion aids,mold release agents and similar materials. Consequently the surface ofthe sleeve 12 should be cleaned of these foreign materials before beingbrought into contact with the uncured resin, since the foreign materialsthemselves may well have a high degree of affinity for the peroxidecuring agent and thus detrimentally affect the polyester cure.

While the preferred embodiment is directed to use on asbestos-cementpressure pipe, the invention can be used in conjunction with other pipein other environments.

What is claimed is:
 1. In a method of forming a coupling for pipeincluding:(a) forming a rubber sleeve having gasket means for sealinglyengaging the ends of adjacent lengths of pipe; (b) supporting saidsleeve on a mold mandrel; (c) forming a fiber reinforced resin bodyaround and in intimate contact with said sleeve; and (d) curing saidbody around and in intimate contact with said sleeve at elevatedtemperatures to form a coupling capable of supporting said liner duringsealing engagement with said pipe, the improvement comprising:supporting said sleeve on said mandrel by stretching said sleeve by apredetermined degree in order to place said sleeve under acircumferential stress so as to reduce the cross-sectional area of saidsleeve to substantially compensate for the displacement of the thencuring resin body caused by the thermal expansion of said sleeve duringsaid curing step whereby dimensional discrepancies are substantiallyprevented.
 2. A method as set forth in claim 1 wherein step (a) furtherincludes providing a predetermined length of extrusion having across-sectional shape defining said gasket means along each longitudinaledge joined by an integral liner portion and bonding the ends of saidextrusion to form said sleeve.
 3. The method as set forth in claim 2wherein said extrusion is of a peroxide cured EPDM rubber and said resinbody comprises glass fibers in a polyester matrix.
 4. The method as setforth in claim 2 and further comprising providing said cross-sectionalshape of said extrusion with a gasket portion along each edge andjoining each said gasket portion to said liner portion by an integralmember having the shape of a radially inwardly open groove.
 5. Themethod as set forth in claim 4 further including: providing a mandrelhaving a substantially right circular cylindrical surface thereon,attaching to one end of said surface an end ring having an outwardlyfacing surface substantially defining a molding surface for said fiberreinforced resin body, installing a flexible support into the innercircumference of said sleeve such that said support is positionedbetween said gasket portions and is substantially parallel to andsupports said liner, and stretching said sleeve with said polymericsupport installed therein onto said cylindrical surface of said mandrelso as to bring one of said gasket portions into substantially continuouscircumferential contact with said end ring, and installing a second endring on the other end of said cylindrical surface so that the other ofsaid gasket portions is brought into substantial circumferential contactwith said second end ring.
 6. The method as set forth in claim 4 furtherincluding: providing in said sleeve a generally right circularcylindrical support means positioned between and abutting against bothsaid gasket portions having undercut portions extending around itscircumferential edges; stretching said sleeve with said support meanspositioned therein onto said mold mandrel in such a manner that saidsupport means substantially fills the area between said liner and saidmandrel: after forming said fiber reinforced resin body around saidsleeve on said mandrel, permitting said sleeve to reach a temperature ofabout 340° F. to 360° F.; permitting said gasket portions to thermallyexpand such that at least part of said gasket portions expand in anaxial direction into said cut-out portions of said support means.
 7. Themethod as set forth in claim 2 and further comprising providing saidcross-sectional shape of said extrusion provided therein includes onsaid gasket means generally axially extending circumferentialprotrusions, and winding resin impregnated fiber glass reinforcingfilaments circumferentially around said sleeve such that at least onecircumferential wrap of said filaments is positioned radially inward ofsaid protrusions and in intimate contact with said gasket means.
 8. Themethod as set forth in claim 1 wherein step (b) further includesstretching said sleeve onto and around said mandrel so as to increasethe average diameter of said sleeve by about 2%.